There are few more hostile environments for electrical cable than an oil well. Yet, it is frequently necessary to utilize down-hole cables in oil wells for such equipment as submersible pumps, well-logging and various other functions.
Among the potentially destructive elements which these cables are subjected to are gas and hydrostatic pressures which may exceed 5000 psi in wells of depths of the order of 10,000 feet. These pressures cause gas and fluids to permeate the cable insulation, and cables which are removed from such wells frequently exhibit embolisms and ruptures which cause cable failure.
When cables are exposed to such fluid permeation for long periods, electrical degradation of the insulation results. One expedient which has been used to partially alleviate this condition is a low permeability insulation. Such insulation allows the gases collected in the cable to be vented when the cable is removed from the high pressure environment of the oil well.
Another technique for preventing the ingress of gas and moisture is to cover the cables with material such as lead. However, such coverings are rather easily embrittled or otherwise damaged by flexing and handling. Some success has been achieved by utilizing a low-swell type of oil-resisting nitrile rubber compound combined with a metal cladding. Even if openings develop in the metal cladding, the combination remains somewhat effective.
In addition to gas permeation problems, the fluids found in wells and in the drilling materials which are used are frequently corrosive or abrasive to a high degree. Various materials have been adopted to prevent the ingress of fluids in oil well cables, and they range from conventional armoring techniques to internal pressurization to resist such inflow of destructive fluids from the well. One technique is to wrap metal of plastic tape over the layer of insulation which in turn encloses the active conductor. The metal tape is then usually covered with an insulating material which in turn is protected by a jacket of armor. Despite the use of such complex and relatively expensive structures, chemical and electrolytic corrosion of the metallic members frequently occurs, especially when cable is left in a well for prolonged periods. This corrosion usually begins at the outer layer, but soon works its way through the metal to cause breakdown of the cable.
It is a primary object of the present invention to improve oil well cable.
It is another object of the present invention to increase the resistance to gas permeation of electrical cable.
It is another object of the present invention to increase the resistance to moisture permeation of electrical cable.
It is another object of the present invention to increase the resistance of electrical cable to chemical and electrolytic corrosion.
It is a further object of the present invention to lengthen the life and improve the performance of down-hole cable.